This invention relates to a process for the removal of SO.sub.2 from a stack gas, particularly the stack gas of a sulfuric acid plant. More particularly, this invention relates to such a method for the removal of SO.sub.2 from a stack gas, employing an absorptive medium which provides for cyclic absorption/desorption of SO.sub.2. The present invention also relates to such cyclic process and the absorptive medium and method of producing the same.
Due to the ever increasing concern about air pollution, attention has been directed in recent years toward the development of processes to reduce the pollutants introduced into the atmosphere from various chemical processes. One of the chief pollutants to the atmosphere is sulfur dioxide which is present in the stack or flue gases from various chemical sources. For example, SO.sub.2 is the chief pollutant vented to the atmosphere in the production of sulfuric acid. Also, combustion fumes, particularly those from power plants fed with fuel oil or other sulfur containing fuels or from incinerators from which sulfur containing compounds are burned contain substantial proportions of SO.sub.2.
Taking as an example the pollutant emissions from a sulfuric acid plant, the emissions, principally SO.sub.2, depend upon a number of factors such as plant design, skill in operation of the plant, efficiency of the catalyst used in sulfuric acid production, completeness of recovery operations and the ability of special devices and systems to reduce the emissions. Most of the work which has been carried out recently has been directed toward optimization of this last factor, i.e. the development of an effective system which can treat the emissions from the sulfuric acid plant or similar plant to reduce the amount of pollutant which is eventually vented to the atmosphere. While substantial work has been carried out, the development of an efficient and economical commercial process is still seen as a desirable end, not fully achieved prior to the development of the present invention.
Sulfuric acid is typically produced in a sulfuric acid contact plant, although some sulfuric acid is produced in chamber plants. Each type produces a stack gas or flue gas with considerable SO.sub.2 impurities. The contact process for the production of sulfuric acid generally involves the continuous combustion of sulfur in a combustion chamber, referred to as a sulfur burner or furnace. Combustion air for the furnace is usually taken directly from the atmosphere into a blower and discharged to a drying tower with 93 to 99% sulfuric acid used as the drying agent. The dry air which then contains about 3 mg. water per cu. ft. goes directly into the sulfur burner for the combustion of the sulfur. Generally, contact plants of this type operate at pressure of 2 to 6 p.s.i.g. depending on the plant design, rate of operation and efficiency of the equipment. This pressure gradient decreases as the gas passes through the plant until it is substantially atmospheric at the exit stack.
Generally, the gas leaving the combustion chamber contains 8 to 11% sulfur dioxide by volume. Any hydrocarbons which are present in the sulfur fuel are burned to carbon dioxide and water. Generally, the gases from the combustion chamber are cooled and filtered to remove dust.
The cooled sulfur dioxide gas is then introduced into a solid catalyst converter, with the temperature of the incoming gas being predetermined based upon the quantity and quality of the catalyst employed and the composition and flow of the sulfur dioxide gas. This catalyst converter is, of course, utilized to convert the sulfur dioxide to sulfur trioxide. The amount of sulfur trioxide leaving the converter is approximately the same as that of the entering sulfur dioxide. In addition, the exit gas contains oxygen, nitrogen, unconverted sulfur dioxide and traces of moisture and carbon dioxide. The moisture results from the incomplete drying of the combustion air or from burning of hydrocarbons in the sulfur. Trace amounts of carbon dioxide are also introduced from the hydrocarbon combustion.
Generally, the exit gas is cooled prior to introducing this stream into an absorption tower where the sulfur trioxide is absorbed countercurrently in a circulating stream of 98 to 99% sulfuric acid. The sulfur trioxide combines with the water in the acid and forms more sulfuric acid. In the absence of a special device or system to thereafter treat the unabsorbed gas, the gas is vented directly to the atmosphere. It is this gas, containing uncoverted sulfur dioxide, which creates substantial pollution problems.
The removal of sulfur dioxide from a gas stream has been proposed and achieved by both chemical and physical methods, using both liquid and solid substances. One difficulty, however, in developing an economic process for the removal of SO.sub.2 is that in most instances large volumes of gas containing low concentrations of SO.sub.2 must be processed quickly and efficiently to achieve the necessary absorption. This requires the use of a great amount of absorbent material.
Because of the economics involved, commercial processes for SO.sub.2 removal have developed along two lines, the first is one in which the absorption of SO.sub.2 into some liquid or solid system provides a saleable by-product. The other is a cyclic absorption/desorption process in which the liquid or solid absorbent can be regenerated and reused. For the most part, however, the processes proposed for the removal of SO.sub.2 and conversion to a by-product are expensive and the costs of removal are rarely is ever recovered in converting the SO.sub.2 to a useful by-product. This is particularly true where the amount of SO.sub.2 in the stack or flue gas is too low to make sulfur recovery operations feasible. As a result, more recent attention has been directed towards attempts to economically remove SO.sub.2 from the stack or flue gas without the benefit of by-product recovery, such as through a cyclic absorption/desorption process.
The greatest percentage of processes which are proposed for the removal of SO.sub.2 involve contacting the sulfur dioxide containing gas stream with an aqueous absorbent stream which typically contains materials which chemically or physically react with the SO.sub.2 in order to absorb the same into the liquid solution. A widely proposed and examined system involves the use of a solution of an alkaline reagent such as an ammonium, alkali metal or alkaline earth metal, hydroxide or carbonate which reacts with the SO.sub.2 in the gaseous stream to produce a rich absorbent stream containing the corresponding sulfite and/or bisulfite compound. While this simple concept of scrubbing an SO.sub.2 -containing gas with an aqueous absorbent containing an alkaline reagent has many advantages, including simplicity and fairly high effectiveness, widespread adoption of this technique for the elimination of SO.sub.2 pollution has been greatly inhibited due to the lack of an economical and efficient continuous regeneration procedure for the rich absorbent stream.
While various cyclic processes have been proposed and certain cyclic processes have been utilized for SO.sub.2 removal, the development of an economical and efficient absorption process coupled with an economical and efficient desorption process has been a problem which continues to plague environmentalists concerned with SO.sub.2 pollution. This problem, however, has now been eliminated through the process of the present invention.